The present invention relates to the transfer and support of wet cellulosic webs between two moving elements in a paper machine. More specifically, the present invention relates to the detachment of moving wet cellulosic webs from a press roll or web supporting belt to another moving element in the press section or dryer section of a paper machine.
In the fabrication of paper, a suspension of cellulosic fibers, referred to as a furnish, is spread on one or more moving forming fabrics or carriers and the bulk of water drained away. This cellulosic web or sheet, which is initially weak and wet, is transferred onto a press felt which carries it into a press nip formed by two press rolls. The mechanical compression between the two press rolls compacts the web and eliminates part of the water from the wet web. The web usually leaves the press nip adhering to one of the press rolls, and must be peeled from the roll before it can be transferred to the next section of the paper machine. Paper machines generally have one to four presses in the press section followed by a dryer section with heated dryer rolls, to evaporate most of the water remaining in the pressed web. In the fabrication of some paper grades, the dry web is moistened by the application of an aqueous suspension of sizing agents. This occurs in a size press after a first drying stage, and the moist sized paper is then again transferred to a second dryer section where it is dried for a second time.
While in the different sections of the paper machine, the wet cellulosic web is usually supported by a pervious belt such as forming fabric, press felt, and drying fabric, or by other means such as a press roll. A mechanical support is often unavailable during web transfer between the individual moving elements of the machine. Thus during web transfer there is an increased danger of the web or sheet breaking, especially if it is moist and the machine operates at high speed. To reduce the danger of sheet breaks it is sometimes necessary to reduce the machine speed, even though this leads to a decrease in production. The danger of sheet breaks is sometimes reduced by the addition of chemicals or by increasing the proportion of a stronger, but more expensive, component such as chemical pulp or long fibre pulp in the furnish or initial fibre mix.
The most critical areas of sheet transfer are from the forming section to the press section, between the consecutive presses in the press section, and between the last press in the press section and the first roll in the dryer section. In all of these transfer areas, the web or sheet is still wet and thus is comparatively weak. Several methods have been used for transferring the sheet at these areas. In one method, the sheet is pulled unsupported from one element to the next through a so-called "open draw". The wet sheet in the open draw is unstable at high speeds and reacts to small variations in the process, sometimes having a tendency to oscillate or flutter. An excessive sheet flutter can cause deformations and wrinkling of the web and reduce the product quality or completely break the sheet and interrupt production. Thus, paper machines with an open draw between the former and the first press rolls usually operate at speed below 750 meters per minute.
All the machines operating at high speeds, that is to say in excess of 1,000 meters per minute, provide a continuous support of the web from the former to at least the first nip in the press section. On machines with multiple roll press arrangement, the web is continuously supported up to the second or third press nip. However, on all present paper machines, the sheet passes through an open draw as it is peeled from the roll of the last press.
In the open draw method of transfer, the reduction of excessive sheet flutter and stabilization of the web is sometimes achieved by increasing the tension in the web by increasing the speed differential between the machine elements. The tension required to peel the web and to stabilize it in the open draw transfer may, in some instances, be sufficiently great to cause a break in the web and even if it does not break, a high tension can permanently stretch the web and, therefore, make it more susceptible to breaks during the subsequent operations on the paper machine. This reduced extensibility is preserved even in the finished product and can lead to an increased number of paper sheet breaks during converting or printing operations.
The speed of paper machines has been increasing throughout the history of papermaking; however, not always at an equal rate. Several times in the past, the machine speed reached a level at which a technical innovation had to be introduced to make a speed increase feasible. For example, the open-draw between the former and the first press subjected the weak paper web to failure stresses and limited the machine speed to a maximum of about 750 m/min. This problem has been eliminated in the 1950's by the introduction of a so-called suction pick-up. In machines with this system, a vacuum roll wrapped by a press felt removes the moist paper web from the forming fabric and carries it into the first press. Paper machines in which the first open draw occurs only after the first press can reach speeds of up to about 800 m/min. Further speed increase was achieved in the 1960's by the introduction of three-roll inclined presses which closed the open draw between the first and second press. Machines with the open draw after the second press have since reached speeds up to about 1100 m/min. In the 1970's further speed increase was made possible by the introduction of four-roll, three-nip presses which closed the draw between the second and third press. Machines with this press arrangement are now operating at speeds greater than 1200 m/min. In the 1980's so-called "unifelted" dryer sections were introduced, which eliminated open draws between the wet-end dryers. In the machines built in recent years, all the open draws in the dryer section have been closed and the maximum speed of such machines approaches 1500 m/min. The open draw between the last press and the dryer section is the only remaining open draw of a modern paper machine. This last open draw must be eliminated before any further increase in machine speed can be achieved.
The need to improve the web transfer from the press section to the dryer section is widely recognized by the industry and much effort is presently being devoted to resolving this problem. As an example of this effort, we would like to list several recent patents which address the web transfer, in a different manner than our invention: U.S. Pat. No. 4,016,032, issued Apr. 5, 1977; U.S. Pat. No. 4,943,351, issued Jul. 24, 1990; U.S. Pat. No. 4,543,160, issued Sep. 24, 1985; and CA 1223143, published Jul. 23, 1987. In spite of all this effort, the problem of web transfer from the press section to the dryer section has not yet been resolved and, to our knowledge, no machine is completely free of open draws.
The importance of the open draw closure is illustrated by the following two examples. A major Canadian supplier of paper machines has prepared a proposal for the installation of a web-transfer system according to our invention for paper mills of two major paper manufacturers. The proposals are presently under consideration by the mills which belong to the world's leading pulp and paper companies.
It has been suggested that the vacuum transfer roll had been previously described in GB 1078634 (published in August 1967). In that patent, the vacuum roll was a press roll, while our invention teaches the application of a no-load transfer roll which only lightly contacts the press roll.
The suction presses such as that described in GB 1078634 cannot support a sufficiently high nip load to provide the desired degree of water removal and, therefore, today their application is only limited. For example, the latest survey of Canadian newsprint machines (I. I. Pikulik and T. H. Owston, Annual Meeting TS CPPA, Montreal, Canada, January 1992) revealed that none of the third presses had a suction roll. However, a main application of our invention is to close the open draw between the third presses and dryers.
The reduction of press nip pressure to optimize it for transfer would not lead to a desirable result. The main function of the press section is to increase the web solids content by mechanical means. The web solids content increase is affected by several parameters and, in particular, is proportional to the pressure in the press nip.
The nip pressure normally increases going from the first to second to third (and possibly to a fourth) press. In the work according to the instant invention, we have found that the application of low pressure in the last nip has a detrimental effect. For example, if the pressure in the last nip is lower than the pressure in the preceding nip, then the last nip removes no water from the web and furthermore some water is transferred from the press felt into the web. Such a result is contradictory to the main objective of web pressing, and is clearly unacceptable. During the work in accordance with the instant invention, it was discovered that if the nip pressure is almost completely eliminated and replaced by only light contact between the transfer roll and the press roll, web rewetting does not occur. This observation can be explained in the following manner. When the nip pressure is greater (such as in a lightly-loaded press nip) the felt is compressed sufficiently to become saturated with water, which is then transferred by capillary forces to wet paper. No such saturation of the nip occurs at very low nip loads. This observation was made during long and extensive research on pressing conducted in connection with the instant invention. The mechanism of rewetting by the felt is not known and would not be obvious to a person knowledgeable in the art of papermaking. Furthermore, if a press roll is used as a transfer roll, the paper sheet rewetting would continue even outside the nip, as long as the sheet adheres to the felt. The sheet moisture content could be increased by 3% or more, a severe penalty.
Another method of transferring a web from a pervious carrier or belt such as a forming fabric to another pervious carrier such as a press felt is with the assistance of a drilled roll equipped with a vacuum chamber. Most high speed paper machines use such a vacuum pick-up system to transfer the web from the former to the first press roll. In a vacuum pick-up system, however, a suction roll can only efficiently transfer a web from a pervious carrier to another pervious carrier. Press rolls are generally solid rolls and thus a vacuum system such as a suction roll cannot by itself initiate peeling of a web from a solid press roll or even an impervious belt. In the case of a press roll, the web normally adheres better to the smoother and less pervious surface.
Since separation of the leading edge of a wet web from a press roll or web supporting belt is difficult to achieve, paper machines are commonly initially threaded with only a narrow band of the web which is sometimes referred to as a "tail". When this narrow band has been successfully threaded through the length of the machine, it is gradually widened until the full width of the paper machine is achieved. This narrow band of paper is initially very weak because it is so narrow and air currents in fast running machines frequently cause the narrow strip to break, thus prolonging the start-up procedure. All the paper produced during machine start-up is unusable and must be recycled. If the machine threading time could be shortened and the machine threaded with the full width of the sheet or web, then production losses would be decreased and a higher efficiency achieved.
Undesirable materials, which generally represent fractions of cellulosic fibres, often adhere to various paper machine rolls such as press rolls, dryers or callender rolls, and are commonly removed by so-called "doctor blades" which have sharp edges positioned in close proximity to the surfaces of the machine rolls and scrape off the web and fibres adhering to the roll. The web removed in this manner is generally densely crimped or creped and cannot be converted into a smooth paper. Creping of a web by a doctor blade may be applied commercially to produce soft and bulky tissue paper used primarily for hygienic products. For high bulk and softness, it is desirable that the tissue paper have regularly and densely spaced creped ridges. Good creping requires a sharp doctor blade and an optimal contact angle between the blade and the impinging web. Canadian Patent number 1,044,459 and Japanese Patent Number 43160 disclose methods of creping by using a hollow doctor blade from which a flat jet of compressed air is blown from a location adjacent the blade. Both of these patents have as a primary objective, the reduction of the wear of the roll and the blade through a reduction or elimination of blade contact with the roll. These hollow doctor blades were designed for production of creped paper rather than for initiation of the transfer of a wet cellulosic web in the press section or immediately prior to the dryer section. Because creping occurs when a web is removed by a blade from a smooth surface, such as a press roll, doctoring has not been used as a means of transfer for wet cellulosic webs to produce paper which requires a smooth surface.